Nickel and cobalt are generally found together in natural-occurring minerals and, because conventional ore dressing methods do not effect a separation of the two, both metals generally appear together in solutions resulting from the leaching of nickel and cobalt-containing materials, such as leached oxide ores, sulfide concentrates and the like.
One method for the recovery of nickel and/or cobalt from oxide ores, such as lateritic limonitic ores, resides in using aqueous sulfuric acid as the leachant at high temperature under elevated pressure, the raw ore being prepared in a finely divided state, then forming a slurry thereof at about 10% to 20% solids which is thereafter concentrated by settling and decanting in thickeners to produce an underflow having a concentration of about 40% to 50% solids. The concentrated slurry is heated in an autoclave by means of direct high pressure steam to a high temperature at which the leaching or other recovery treatment is carried out, usually above 400.degree. F. (205.degree. C.), e.g. about 475.degree. F. (246.degree. C.), at a pressure of about 525 psig in the presence of sulfuric acid to solubilize the nickel and cobalt present in the slurried ore. Following leaching in the autoclave, the leached pulp is cooled and preferably washed by countercurrent decantation and the resulting acid leach liquor then treated with a neutralizing agent [Mg(OH).sub.2, coral mud or the like] to raise the pH to, for example, 2.5 to 2.8 for the sulfide precipitation of nickel and cobalt. The leach liquor is brought to a temperature of about 250.degree. F. (122.degree. C.) and the nickel and cobalt precipitated as sulfides with H.sub.2 S at pressures of up to about 150 psig, using nickel sulfide as seed material.
The sulfide precipitate is washed and thickened to about 65% solids and then oxidized at about 350.degree. F. (177.degree. C.) and a pressure of about 700 psig in an autoclave in 1% sulfuric acid. Ammonia is added as a neutralizing agent to the nickel-cobalt solution to raise the pH to a level (e.g. 5.3) using air as an oxidant, to precipitate any iron, aluminum or chromium carried over as an impurity during leaching. After separating the solution from the precipitate, any copper, lead or zinc present therein is removed by precipitation as a sulfide, using H.sub.2 S as the precipitant, the solution being first adjusted with acid to lower the pH to about 1.5. The sulfide precipitate is then separated from the solution and the solution adjusted with ammonia to prepare it for the recovery of metallic nickel. The adjusted nickel feed solution containing about 40 to 50 grams per liter of nickel and some cobalt is reduced with hydrogen in an autoclave at about 375.degree. F. (190.degree. C.) and 650 psig using nickel powder as seed material, the barren liquor remaining going to cobalt recovery using known methods. However, some of the cobalt appears in the reduced nickel product.
Among the methods proposed and/or commercially used for separating cobalt from nickel is the method of separating cobalt from aqueous nickel-cobalt sulfate solutions by means of nickelic hydroxide, and the subsequent separation of cobalt from nickel in the resulting precipitate by means of the pentammine process described in U.S. Pat. Nos. 2,767,053 and 2,767,054, the cobalt in the precipitate being in the cobaltic state.
In one embodiment, the cobaltic precipitate which also contains nickel is solubilized as cobaltic pentammine and nickelous ammine by dissolving the cobaltic precipitate in an ammonium sulfate solution containing at least about 100 grams per liter of (NH.sub.4).sub.2 SO.sub.4 at least about 50 grams NH.sub.3 per liter (gpl) at a temperature ranging from about 80.degree. C. to 120.degree. C. under a pressure of at least about 20 psi gage. This solution is then acidified to a pH of about 1.5 to 3.0 and cooled to produce a nickel-ammonium sulfate precipitate highly enriched in nickel.
While the foregoing technique is useful in the extraction and recovery of cobalt from mainstream nickel-cobalt sulfate solutions, it has certain disadvantages. For example, the cost of reagents for forming the pentammine is high.
Another method which has been employed to separate cobalt from solution is to use nickelic hydroxide to oxidize the cobaltous ion to the cobaltic state and then cause the precipitation thereof as cobaltic hydroxide.
It is known according to Czech Pat. No. 100,929 (Sept. 15, 1961) to purify sulfate electrolytes by ozone injection at 20.degree. to 100.degree. C. wherein certain metal ions are oxidized and caused to hydrolyze from solution. Since free acid is generated during hydrolysis, an hydroxide or a carbonate of an appropriate basic metal is added during ozonation. Copper, lead, chloride ion, organic compounds, among others are simultaneously removed from solution. Similar processes for removing metals and non-metal impurities are disclosed in Czech Pat. Nos. 102,895 (Mar. 15, 1962) and 106,524 (Feb. 15, 1963).
In a paper entitled "Application of Ozone to Nickel Salt Production" by A. D. Tolstoguzov and Yu. N. Lozitskii [Tsvetnye Metally, 10, p. 25(1973)], ozone is used to precipitate cobalt from nickel sulfate solution using either nickel or sodium carbonate for neutralization. However, no mention is made of the presence of ammonium ions which is known to have an adverse effect on the reaction.
Prior ozonation processes for nickel-cobalt separation avoided solutions containing nitrogen in the -3 oxidation state. For example, ozone oxidizes ammonium hydroxide to ammonium nitrate at atmospheric pressure and temperature according to the following reaction: EQU NH.sub.3aq +40.sub.3g .fwdarw. NO.sub.3.sup.- +40.sub.2g +H.sup.+.sub.aq +H.sub.2 O (1)
the higher the aqueous ammonia concentration, the higher the solution pH, and the higher the ozone concentration in the oxygen carrier gas, the further the foregoing reaction progresses. Thus, the presence of the ammonium ion, for example, as ammonium sulfate, in nickel-cobalt solutions interferes with the ozonation rejection of cobalt by consuming ozone before it can oxidize cobalt (or nickel, which in turn oxidizes cobalt).
We have found, however, that we can overcome the foregoing limitation. This limitation is a serious one when one considers that present hydrometallurgical processes for nickel recovery generate solutions containing ammonia from which cobalt must subsequently be removed.